Optical spectrometers are instruments that disperse light radiation into spectral patterns for analysis. Such instruments are used for various purposes, for example to effect and analyze spectral lines characteristic of the atomic elements in a sample. For an accurate quantitative analysis or detection of minute quantities of elements, the spectrometer must have a high degree of precision.
One type of spectrometer used for analysis of inorganic samples is an atomic emission spectrometer that utilizes an induction coupled plasma ("ICP") source of light radiation. A nebulized sample material is injected into the plasma where it becomes disassociated into atoms which are excited in the plasma so as to emit radiation including the spectral lines. An example of such an induction plasma system is disclosed in U.S. Pat. No. 4,766,287 (Morrisroe et al).
A polychromator in a spectrometer disperses the radiation into a band or multiplicity of wavelengths or spectral lines that are detected. An example of a precision polychromator is an echelle system with tandem, crossed dispersion units to produce a two-dimensional display of spectral lines as disclosed in U.S. Pat. No. 4,820,048 (Barnard) of the present assignee. The spectral lines are focussed onto a detector consisting of a two dimensional solid state charge transfer device that produces signals proportional to the intensity of the corresponding lines. A computer processes the signal information, corrects for background, applies calibration, and displays the results in the form of concentrations of atomic elements in the sample.
As illustrated in the aforementioned U.S. Pat. No. 4,820,048, a crossed disperser type of spectrometer may be configured to detect in several spectral ranges, particularly visible and ultraviolet light ranges. The first dispersion element, a diffraction grating, is common for both ranges. For the ultraviolet range, a second grating, having grating lines perpendicular to those of the first, reflects and further disperses the radiation which is focussed to a first detector. For the visible range, the second grating has a central hole herein, and radiation passing through the hole is collected and cross dispersed by a prism and focussed to a second detector. Both detectors are two dimensional types, and signals therefrom are led to the common processing unit. Thus this type of spectrometer, although having high precision for two spectral ranges, entails the expense, complexity and bulkiness of two separate optical trains including dispersers and detectors for the second of the crossed dispersion units. The two ranges need to be detected separately because they otherwise are too widely spread spatially by the dispersers for convenient detection by a single detector.
Components in optical systems can introduce aberrations that distort images. In the case of the above-described spectrometer, a spherical mirror that focusses the radiation from the second grating to the detector creates inherent geometric aberrations in the focussing of the image of the inlet slit of the system. The predominant type is known as spherical aberration that results in focussing of off-axis rays into a slightly different plane than the rays near the rotational axis. This may be corrected by replacing the spherical mirror with a parabolic mirror. However, such mirrors are costly and generally limited to a small field of view which is inefficient in echelle spectrometers.
Another means for correcting for spherical aberration is a Schmidt element. In one form a refracting plate of glass or plastic having a thickness that varies with radius is inserted in the beam. Another type utilizes an additional reflector with a curvature that provides the correction. In either of these cases, the corrector introduces an extra element into the system, with attendant radiation losses, background increases and added bulkiness, complexity and cost. In a third case, as indicated in the aforementioned U.S. Pat. No. 4,820,048, the second grating may be provided with a non-planar surface to function as the Schmidt corrector. Such a corrector grating is disclosed in U.S. Pat. No. 3,521,943 (Kelderman).
An object of the invention is to provide an improved optical spectrometer for detecting spectra in separate spectral ranges. A particular object is to provide a spectrometer having a novel means for effecting spectra in separate spectral ranges. A further object is to provide such a spectrometer having crossed dispersion elements in which the second element allows for selective detection of spectra in the separate spectral ranges. Another object is to provide crossed-disperser spectrometer having a single optical train for detecting spectra in separate spectral ranges. Yet another object is to provide such a spectrometer having for the second dispersion element a grating that corrects for aberrations.